This invention relates to an ink jet recording element, more particularly to an ink jet recording element which contains thermally compliant composite particles having a core-shell structure.
In a typical ink jet recording or printing system, ink droplets are ejected from a nozzle at high speed towards a recording element or medium to produce an image on the medium. The ink droplets, or recording liquid, generally comprise a recording agent, such as a dye or pigment, and a large amount of solvent. The solvent, or carrier liquid, typically is made up of water, an organic material such as a monohydric alcohol, a polyhydric alcohol or mixtures thereof.
An ink jet recording element typically comprises a support having on at least one surface thereof at least one ink-receiving layer. The ink-receiving layer is typically either a porous layer that imbibes the ink via capillary action, or a polymer layer that swells to absorb the ink. Swellable hydrophilic polymer layers take an undesirably long time to dry so that porous layers which dry more rapidly are generally regarded as superior. Ink jet recording elements may contain several layers on the support. Typical two layer constructions have either an uppermost ink transporting layer in combination with a ink retaining underlayer, or an uppermost ink image capture layer in combination with an underlying ink vehicle sump layer.
Porous layers typically contain an easily wettable but water insoluble refractory inorganic pigment as well as a binder. Typically these refractory inorganic pigment particles are comprised of either silica or alumina. The high loading of these easily wettable but refractory particles and the presence of numerous voids, which are essential to the rapid ink absorption of the layer, presents a problem in that the many interfaces presented by such a layer leads to light scattering and results in insufficient gloss. To reduce the scattering of light and thereby improve gloss, porous layers are often comprised principally of colloidal, i.e., less than 0.5xcexc, particles. However, these particles are difficult to coat without cracking of the coated layer. Thus it is difficult to achieve high gloss with refractory particles larger than about 0.5xcexc, and conversely it has proven difficult to coat a non-cracking layer with refractory particles smaller than about 0.5xcexc.
U.S. Pat. No. 5,576,088 relates to an ink jet recording sheet having at least one ink-receiving layer and a gloss-providing layer consisting of a synthetic polymer latex binder and a pigment, at least 70% by weight of which is colloidal particles. The gloss-providing layer may be calendered or pressure contacted to a heated specular roll immediately after coating to further enhance the gloss. However, there are problems with this recording sheet in that the use of organic particles decreases the releasability of the gloss-providing layer from the specular roll. Further, calendering the layer prior to imaging decreases ink penetrability. In addition, the above layers have a high loading of colloidal particles so that the layers are prone to cracking due to high drying stresses.
U.S. Pat. No. 5,472,773 relates to a coated paper comprising a substrate with a surface layer of colloidal aggregates alumina crystals (or psuedo-boehmite) and a binder having a specular gloss at 60xc2x0 of at least 30%. However, there are problems with this coated paper in that this gloss level is less than desirable for high quality imaged substrates and preparing the coated paper requires a costly and complex layer transfer technology.
EP 0 813 978 A1 discloses a porous ink jet recording sheet having solid fine particles in a hydrophilic binder with oil drops to reduce layer brittleness and cracking. However, there is a problem with this element in that oil drops can be exuded from the coating to give an unwanted oily surface feel and diminished gloss.
It is an object of this invention to provide an ink jet recording element having a porous top layer that can either transport or retain an ink image, and which can be heat and pressure treated to a high gloss despite the refractory nature of incorporated inorganic pigment particles. Another object of the invention is to provide a glossable ink jet recording element which can be coated and dried without cracking and yet maintain good ink absorptivity.
These and other objects are provided by the present invention which relates to an ink jet recording element comprising a support having thereon, in the order recited, at least one base layer comprising a hydrophilic or porous material and a porous top layer capable of either retaining or transporting an ink image, the porous top layer comprising a polymeric binder and thermally-compliant core-shell particles, the particle-to-binder ratio being between about 95:5 and 50:50, and wherein each thermally-compliant core-shell particle has:
a) a shell of inorganic colloidal particles, and
b) a core of a thermoplastic polymer,
the particles having a particle size between about 0.5 xcexcm and about 10 xcexcm, the polymeric core having a softening point of greater than about 50 xc2x0 C., and the weight ratio of the shell of the inorganic colloidal particles to the thermoplastic core being from about 1:5 to about 1:99.
By use of the invention, an ink jet recording element is provided which has a high gloss, does not crack and has good ink absorptivity.
The composite thermally-compliant core-shell particles used in the invention may be prepared by several procedures including evaporative limited coalescence, as described in U.S. Pat. No. 4,833,060 and limited coalescence, as described in U.S. Pat. No. 5,354,799, the disclosures of which are hereby incorporated by reference. In both of these preparations, the shell is formed in-situ by a promoter during the preparation of the particle. Alternatively, the shell may be assembled via the layer-by-layer technique on preformed particles as described in xe2x80x9cElectrostatic Self-Assembly of Silica Nanoparticle-Polyelectrolyte Multilayers of Polystyrene Latex Particlesxe2x80x9d in the Journal of the American Chemical Society, vol. 120, p. 8523 (1998).
The thermoplastic core polymer employed in the invention has a softening point greater than about 50xc2x0 C., and preferably between about 50xc2x0 C. and about 120xc2x0 C. A softening point of a polymer can be measured by the Ring and Ball method as described in ASTM E28.
In a preferred embodiment of the invention, the thermoplastic core polymer is a polyester, an acrylic polymer or a styrenic polymer. Examples of these polymers include an amorphous polyester Kao C(copyright) (Kao Corp.), an acrylic polymer such as Carboset 526(copyright) (BF Goodrich Specialty Chemicals), or a styrene allyl alcohol copolymer such as SAA 100(copyright) (Lyondell Chemical Co.).
If the limited coalescence process is used to prepare the composite particles employed in the invention, then monomers and polymerization conditions should be chosen which will polymerize to form a core polymer with a softening point as described above. Suitable monomers include styrenic and vinyl monomers such as styrene, methylmethacrylate or butylacrylate. Mixtures of monomers, such as styrene, butylacrylate and methylmethacrylate may be polymerized to obtain the desired polymer properties.
Suitable colloidal inorganic particles which can be used as the shell material in the invention include colloidal silicas and modified colloidal silicas available from DuPont as Ludox(copyright), and colloidal aluminas such as Dispal(copyright) (Condea Corp.). The size of the colloidal inorganic particles may range from 5 to 100 nm.
The shell of the core-shell particle used in the invention can be further modified, after formation of the primary core-shell particle, to alter a number of particle properties such as the surface charge on the particles. If the upper layer is to be ink retentive, then the surface charge on the particle should be opposite to that of the colorant. If an anionic or negative dye, for example, is the colorant, then the particle charge should be cationic or positive, so as to mordant the dye in the layer. Conversely, if the upper layer is to be ink transporting, then the surface charge on the particle should be rendered either neutral or the same as that of the dye. Surface charge on the particles can be measured via the electrokinetic sonic amplitude (ESA) technique as described in J. Colloid and Interface Science, 173, 406, (1995).
As stated above, the weight ratio of the shell of the inorganic colloidal particles to the thermoplastic core is from about 1:5 to about 1:99, preferably from about 1:15 to about 1:50. The % silica is determined, on a sample washed free of unadhered colloidal silica, using 14-MeV neutron activation analysis to measure the Si content as described in xe2x80x9cActivation Analysis with Neutron Generatorsxe2x80x9d S. Nargolwalla and E. Przybylowicz eds. John Wiley and Sons, Inc. (1973), p. 528.
Also as stated above, the particle size of the core-shell particle used in the invention has a particle size between about 0.5 and about 10 xcexcm, preferably from about 0.9 to about 5 xcexcm. The particle size of the core-shell particle is determined by a Horiba LA-920 Laser Scattering Particle Size Distribution Analyzer (Horiba Instruments, Inc.) and is a volume-weighted mean size.
A core-shell particle having a negative surface charge, by virtue of an adherent layer of a negatively charged colloidal silica, can be rendered neutral or cationic by use of cationic surfactants as described in Colloids and Surfaces, 28, (1987) 159-168 and references contained therein. Water-soluble cationic polymers, such as poly(diallyl dimethylammonium) chloride or cationic colloidal latex particles, can be used to modify the surface charge of the core-shell particle as described in the above-referenced article in the Journal of the American Chemical Society. Core-shell particles having a cationic surface charge by virtue of an adherent layer of cationically charged colloidal silica, can be rendered anionic by similar procedures. Further, the surface charge and wetting properties of the silica shell can be modified by treatment with a variety of silanes as described in Chemtech,7, 766-778 (1977).
The polymeric binder useful in the recording element of the invention is not particularly limited. Any polymer or mixture of polymers, which are film formers and function to bind the particles described above to form a coherent layer on coating, will be useful. Examples of such binders include water soluble polymers such as gelatin, poly(vinyl alcohol), poly(ethylene oxide), poly(2-ethyl-2-oxazoline), cellulosic polymers such as methyl cellulose, emulsion polymers and copolymers such as ethylene-vinyl chloride, poly(acrylates), poly(vinylacetate), polyvinylidene chloride, vinylacetate-vinyl chloride, and aqueous polymer dispersions such as polyurethanes and polyurethane alloys.
As stated above, the particle-to-binder ratio is between about 95:5 and 50:50, preferably between about 90:10 and 80:20. If the particle-to-binder ratio is above the range stated, the layer will not have any cohesive strength. If the particle-to-binder ratio is below the range stated, the layer will not be porous enough to provide a fast dry time.
The base layer or layers, in general, will have a thickness of about 1 xcexcm to about 50 xcexcm, and the top layer will usually have a thickness of about 2 xcexcm to about 50 xcexcm.
If the uppermost layer is retentive of the ink image, then the base layer will act as a reservoir or sponge layer for the absorption of ink solvent. If the uppermost layer is ink transporting, then the base layer will additionally serve to retain the ink image. The base layer may be hydrophilic and swellable or porous. Generally, the base layer is present in an amount from about 1 g/m2 to about 50 g/m2, preferably from about 5.0 g/m2 to about 30 g/m2. Suitable hydrophilic materials include gelatin, acetylated gelatin, phthalated gelatin, oxidized gelatin, chitosan, poly(alkylene oxide), poly(vinyl alcohol), modified poly(vinyl alcohol), sulfonated polyester, partially hydrolyzed poly(vinylacetate/vinyl alcohol), poly(acrylic acid), poly(1-vinylpyrrolidone), poly(sodium styrene sulfonate), poly(2-acrylamido-2-methane sulfonic acid), polyacrylamide or mixtures thereof. Copolymers of these polymers with hydrophobic monomers may also be used. Suitable porous materials for a base layer include, for example, silica or alumina in a polymeric binder, including hydrophilic binders such as those described above.
In a preferred embodiment of the invention, the base layer comprises gelatin which may have up to about 15% of another hydrophilic material such as poly(1-vinylpyrrolidone). In another preferred embodiment, the base layer is porous fumed alumina in a crosslinked poly(vinyl alcohol) binder.
The support used in the ink jet recording element of the invention may be opaque, translucent, or transparent. There may be used, for example, plain papers, resin-coated papers, various plastics including a polyester resin such as poly(ethylene terephthalate), poly(ethylene naphthalate) and poly(ester diacetate), a polycarbonate resin, a fluorine resin such as poly(tetra-fluoro ethylene), metal foil, various glass materials, and the like. In a preferred embodiment, the support is a resin-coated paper. The thickness of the support employed in the invention can be from about 12 to about 500 xcexcm, preferably from about 75 to about 300 xcexcm.
If desired, in order to improve the adhesion of the base layer to the support, the surface of the support may be corona-discharge-treated prior to applying the base layer or solvent-absorbing layer to the support.
Since the image recording element may come in contact with other image recording articles or the drive or transport mechanisms of image recording devices, additives such as surfactants, lubricants, matte particles and the like may be added to the element to the extent that they do not degrade the properties of interest. In addition, the top layer of the invention may also contain other additives such as viscosity modifiers or mordants.
The layers described above, including the base layer and the top layer, may be coated by conventional coating means onto a support material commonly used in this art. Coating methods may include, but are not limited to, wound wire rod coating, slot coating, slide hopper coating, gravure, curtain coating and the like. Some of these methods allow for simultaneous coatings of both layers, which is preferred from a manufacturing economic perspective.
Ink jet inks used to image the recording elements of the present invention are well-known in the art. The ink compositions used in ink jet printing typically are liquid compositions comprising a solvent or carrier liquid, dyes or pigments, humectants, organic solvents, detergents, thickeners, preservatives, and the like. The solvent or carrier liquid can be solely water or can be water mixed with other water-miscible solvents such as polyhydric alcohols. Inks in which organic materials such as polyhydric alcohols are the predominant carrier or solvent liquid may also be used. Particularly useful are mixed solvents of water and polyhydric alcohols. The dyes used in such compositions are typically water-soluble direct or acid type dyes. Such liquid compositions have been described extensively in the prior art including, for example, U.S. Pat. Nos. 4,381,946; 4,239,543 and 4,781,758, the disclosures of which are hereby incorporated by reference.